Pro-Russian separatists search for ammunition at the site of the crash of the Il-76 Ukrainian army transport plane in Luhansk, June 14, 2014. Shamil Zhumatov/Reuters Ukraine's new president vowed to "punish" pro-Russian insurgents on Saturday after they shot down a transport aircraft, killing 49 people in the bloodiest loss for the security forces since the rebellion began.

The Ilyushin-76 (IL-76) was destroyed as it came in to land at Luhansk airport shortly before 1am local time.

Footage from a CCTV camera shows an explosion lighting up the night sky over the city, followed by a second detonation apparently caused by the plane hitting the ground.

There is no sign of anti-aircraft gunfire in the sequence. Footage taken from a second camera then shows a fire burning fiercely on the ground, apparently from the site of the crash outside Luhansk.

This evidence has not been independently verified, but it would be consistent with the plane being destroyed by a shoulder-launched surface-to-air missile.

The IL-76, a four-engined transport aircraft, was carrying 40 Ukrainian soldiers along with nine crew and an undisclosed quantity of military supplies.

The defence ministry in Kiev confirmed the deaths of all on board, accusing "terrorists" of acting "cynically and treacherously" and bringing down the aircraft with "heavy calibre machine guns" - although the footage suggests that a missile is a more plausible explanation.

Petro Poroshenko, the newly elected president of Ukraine, denounced an "act of terror", adding that "those involved" would "certainly be punished".

Pro-Russian insurgents control Luhansk, a regional capital with 430,000 people, along with much of the surrounding area. They occupied the main government building in the city centre in April and turned this into their headquarters.

But government forces held the small airport outside Luhansk, halting all civilian flights and using this runway to fly in troops and supplies.

Mr Poroshenko has pledged to restore the central government's control over the two eastern regions of Donetsk and Luhansk, pressing on with a military offensive designed to crush the rebellion.

But this operation has been dogged by setbacks - and the loss of the IL-76 is the bloodiest so far. At least three military helicopters have also been shot down in previous incidents, including one that was carrying a general. The evidence suggests that rebels are using shoulder launched anti-aircraft missiles - almost certainly supplied by Russia. Their immediate aim around Luhansk would be to disrupt the government's effort to bring in soldiers and materiel.

While making daylight landings in Donetsk or Luhansk, Ukrainian military aircraft routinely fire decoy flares to confuse heat-seeking missiles. The CCTV footage suggests the IL-76 may not have been taking this precaution, perhaps because a night-time approach was judged to be safer.

The loss of the plane overshadowed a rare success for the army's offensive. On Friday, soldiers drove pro-Russian rebels from their last strongholds in Mariupol, the second biggest city in Donetsk region with a population of 500,000.

Aided by a pro-Ukrainian workers' militia, the security forces retook almost all of Mariupol last month. But the insurgents managed to hold out inside the main municipal building in the city centre, which they first occupied in April.

They were finally evicted on Friday after a fierce gunbattle, involving the use of Ukrainian armoured fighting vehicles. The authorities said that five rebels were killed; video footage showed the Ukrainian national flag flying over the municipal headquarters.

Earlier, the true extent of Russian military support for the rebels became clearer when three T-64 tanks appeared inside eastern Ukraine. These obsolete models had apparently been taking from military stores inside Russia and then driven over the border. Government forces later claimed to have destroyed one of the tanks.

The US State Department said that Russia was now supplying the insurgents with heavy weapons, including multiple rocket launchers, as well as the T-64s. "Russia will claim these tanks were taken from Ukrainian forces, but no Ukrainian tank units have been operating in that area," said the State Department. "We are confident that these tanks came from Russia."

The statement added: "We also have information that Russia has accumulated multiple rocket launchers at this same deployment site in south-west Russia, and these rocket launchers also recently departed. Internet video has shown what we believe to be these same rocket launchers traveling through Luhansk."

Denis Pushilin, a leader of the pro-Russian "Donetsk People's Republic", confirmed that the rebels possessed tanks but told Russian state television that it was "improper to ask" where they had come from.

Meanwhile, the security forces discovered a bomb planted near President Poroshenko's office. A container holding five hand grenades and one kilogram of metal nuts was planted at the gates of the street where Mr Poroshenko works - and through which his official car must drive every day.

A source told Reuters news agency that the "powerful device" was found and made safe.

On Thursday, a bomb destroyed a car belonging to Mr Pushilin in central Donetsk, wounding three people. The pro-Russian leader described this as an "assassination" attempt. It is not known whether the two incidents are connected.

But Russia sent a conciliatory signal on Saturday by agreeing to restart negotiations with Ukraine over gas supplies. Arseniy Yatsenyuk, the Ukrainian prime minister, had warned of Russia cutting off Ukraine's gas as early as Monday because of a long-running dispute over the price.

Until February, Russia had supplied Ukraine for $268.5 per 1,000 cubic metres of gas. Then the revolution brought down a pro-Kremlin regime - and Russia immediately raised the price by 80 per cent to $485.

Gazprom, the Russian state energy giant, later suggested that it would accept $385, a sum comparable to the amount it charges other European customers. Ukraine, meanwhile, signalled that it would accept $326 for an interim period of 18 months. Talks will now take place to narrow the gap between the two sides.

Russia cut off supplies to Ukraine in 2006 and 2009. Pipelines serving Ukraine also carry gas to Russia's customers in Europe, meaning that the Kremlin cannot impose an embargo on Ukraine without also cutting off other countries.